So first, always set up a coordinate system. I will choose an origin to be at the starting point of the measurement ( ) and final point to be at . I am choosing the positive x direction to be in the same direction as the acceleration, and we might as well choose that to be horizontal and to the right.
Okay, now we need three equations and put them together. I'm going to choose:
1. (remembering that )
So we have for the average speed:
Solving equation 1 for and putting it into equation 2:
I'll leave the rest to you. You need to factor the numerator, plug in and do some simplifying. If you need help to finish it, just ask.